In order to reduce the bandwidth required for digital transmission of voice frequency signals, it is known to convert an 8-bit PCM voice frequency signal to a 4-bit ADPCM signal. Two such ADPCM signals can be transmitted in the same bandwidth as one PCM signal. Furthermore, in many circumstances such ADPCM signals can be transmitted in place of PCM signals over existing transmission links using existing equipment, which is merely supplemented by ADPCM encoders and decoders for converting between PCM and ADPCM signals at the ends of the transmission links.
For conversion from PCM to ADPCM signals without undue signal degradation, it is known to use an encoder which comprises an adaptive predictor and an adaptive quantizer. The quantizer produces the ADPCM signal from the difference between the incoming PCM signal and a predicted signal produced by the adaptive predictor. Adaptation of the quantizer is controlled by the ADPCM signal, which is also supplied to an inverse adaptive quantizer to produce a reconstructed difference signal which controls the adaptive predictor. Similarly, the decoder comprises an inverse adaptive quantizer to produce a reconstructed difference signal from the ADPCM signal, and an adaptive predictor responsive to the reconstructed difference signal for producing a predicted signal, the latter two signals being combined to produce the decoded PCM signal. Ideally, predictor coefficients of the adaptive predictors in the encoder and decoder are identical at all times. Any departure from this is referred to as mistracking.
In practice, mistracking occurs in that, for example, initialization procedures or transmission errors may cause the predictor coefficients in the encoder and the decoder to have different values at the same time. In order to correct such mistracking, it is known to provide a leakage in each predictor coefficient, whereby in the absence of transmission errors the encoder and decoder adaptive predictor functions converge whereby tracking errors are corrected over a period of time. The leakage must be sufficient that this period of time is not too great, but must also be small enough that the prediction function is not altered to such an extent that the signal is noticeably degraded. These conflicting requirements give rise to a problem in determining an appropriate leakage function.